Cathode ray tube and electron gun therefor



Sept. 16, 1958 J. M. LAFFERTY CATHODE RAY TUBE AND ELECTRON GUN THEREFOR 2 Sheets-Sheet 1 Filed July 14, 1954 m NK Inventor.- James M laffer-ty,

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Sept. 16, 1958 J. M. LAFFERTY CATHODE RAY TUBE AND ELECTRON GUN THEREFOR Filed July 14, 1954 2 Sheets-Sheet 2 Inventor James M- [afferi y )Q/ 4. :7'

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United States Patent @fiice 2,852,716 Patented Sept. 16, 1958 CATHODE RAY TUBE AND ELECTRON GUN THEREFOR James Martin Lalferty, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application July 14, 1954, Serial No. 443,278

3 Claims. (Cl. 315-15) This invention pertains to electron discharge devices such as cathode ray tubes and to electron gun structures therefor.

With the development of large screen and color television tubes, it has become desirable to increase the electron beam currents of such tubes in order that sufficient image brightness may be maintained. Present cathode guns are unable to supply the necessary beam currents Without changing the characteristics of the beam so as to cause undesirable effects, as for example, large spot sizes, undesirably high cut-off voltages, and high cathode current density requirements.

Accordingly, one object of the invention is to provide a cathode ray tube having an electron gun which will produce a higher beam current than has been heretofore possible with conventional electron guns.

A further object of the invention is to provide, for cathode ray tubes, an electron gun in which the functions of beam formation and beam modulation are separated, thus allowing both to be performed more efliciently.

Still another object of the invention is to provide an electron gun capable of providing high beam current without causing the need for unreasonably high beam modulating voltages.

Another object of the invention is to provide an electron gun having a plurality of modulating electrodes which may be operated independently of each other and of the cathode.

According to one feature of the invention, there is provided a cathode ray tube having an electron gun in which the beam forming and focusing functions are separated from the beam modulating function. This separation removes the adverse effect of each of these functions upon the other and allows higher beam currents without introducing undesirable effects.

More specifically, a cathode having a concave electron emissive surface is located within a first apertured anode and surrounded by a focusing electrode. The concave cathode emissive surface allows a greater cathode area to contribute to the beam density. Electrons emitted by the cathode are drawn through the first apertured anode by a concave field established in the vicinity of the cathode emissive surface by the focusing electrode and first anode, thus establishing a small diameter, high density electron beam having a high perveance (which is a constant, dependent upon electrode structure, which relates beam current to accelerating potential). The modulating electrode is located in a field free space between the f rst anode and a second anode, both of which are maintained at the same electrical potential. The modulated electron beam is then further focused by a third anode.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, together with further objects and advan tages thereof, may best be understood by referring to Lil ill)

the following description taken in connection with the accompanying drawing, in which:

Fig. 1 illustrates diagrammatically a cathode ray tube illustrative of the invention;

Fig. 2 illustrates the electron gun portion of the cathode ray tube of Fig. 1;

Fig. 3 illustrates in detail the structure of the cathode and first and second anodes of the cathode gun of Fig. 2;

Fig. 4 illustrates, in detail, an alternative embodiment of the structure of Fig. 2;

Fig. 5 illustrates in graphic form the improved characteristics of a cathode gun constructed according to the invention; and

Fig. 6 illustrates another feature of the invention wherein a plurality of control electrodes is provided.

The conventional type electron gun currently used in cathode ray tubes as for example, television picture tubes, generally comprises a cylindrical cathode with a flat electron emissive surface, an apertured modulating electrode in close proximity to the cathode emissive surface, and an anode in close proximity to the control electrode on the side thereof opposite the cathode. Further accelerating and focusing anodes may be located after the first anode. This type of arrangement has several disadvantages, the first of these being that the area of the cathode from which the electron beam is derived is relatively small, due to the small aperture in the control electrode through which an electric field set up by the first anode may be effective to form an electron beam from electrons thermally emitted from the cathode. With such a small electron emitting surface, the beam intensity may be increased only as the voltage upon the first anode is increased, the limiting factor being the temperature limited emission of the cathode surface. A further disadvantage of this conventional electron gun arrangement is that the negatively biased control electrode, in close proximity to the cathode, adversely affects the shape of the accelerating field in the vicinity of the cathode. Thus, the modulating function of the control electrode adversely influences the beam forming function of the electron gun. On the other hand, the proximity of the electric field due to the first anode to the modulating electrode, adversely influences the modulating function of the electron gun. In other words, the combination of the beam-forming and beam-modulating functions at the cathode surface of conventional electron guns results in two decided disadvantages. The effect of the negatively biased control electrode upon the electric field in the vicinity of the cathode detracts from the electron accelerating function of the first anode. Additionally, the effect of the high positive potential of the first anode upon the field at the control electrode detracts from the modulating function of the control electrode. As a re sult it is not possible, with the conventional electron gun configuration, to focus the beam properly for all values of modulating voltage.

in order to obtain higher beam currents with conventionnl electron gun configurations as described hereinbcforc, the voltage upon the first anode may be increased, thus increasing the number of electrons collected from the cathode emmissive surface. This expedient, however. reficcts adversely upon the modulating function of the tube. For example, as the first anode voltage is increased, the cut-off voltage of the control electrode is also raised. This in turn means that a higher video volt age is required to swing the control grid from the cut- 05 value to zero bias. It is, however, difficult to obtain large voltage outputs from wide band video amplifiers normally used in television receiving sets. Another expedient which may be attempted to increase beam current from a conventional electron gun is to increase the aperture of the control electrode, thus allowing a larger surface of the cathode to contribute to the beam density. This expedient, however, also results in the necessity of higher voltages to cause the electron beam to be cut off, and increases the minimum diameter of the beam which results in a larger spot upon the tube screen, thus reducing the picture resolution.

According to the invention, I have found that the above-mentioned disadvantages of the conventional cathode ray electron gun may be overcome by a number of changes in electron gun structure. Initially, the available surface of the cathode contributing to the beam density may be increased by making the emissive surface of the cathode concave in shape, preferably an arc of a sphere, so that a larger area of the cathode may emit electrons and contribute to beam current. This change in cathode emissive surface configuration while increasing the current density of the beam, still leaves unsolved the problems discussed hereinbefore, of the focusing and modulating functions of the cathode gun adversely affecting each other. This disadvantage may be overcome by separating the beam forming function from the modulating function. 1 have found that this may be accomplished by locating the modulating electrode away from the cathode between a first anode and a second anode which are maintained at the same electrical potential. By this arrangement, the control electrode, which operates at a value from zero bias to a negative value sufficient to cut off the beam, is located in a field free space wherein its negative potential will not afiect the field configuration in the vicinity of the cathode.

The above-described arrangement of electrodes, though helpful in separating the modulating function of the control electrode from the focusing function in the vicinity of the cathode is not, of itself, sutficient to cause the desired increase in electron beam current. In the conventional arragement for cathode guns, the location of the control electrode in the vicinity of the cathode causes the electrons emitted from the cathode, and accelerated under the influence of the potential of the first anode, to be focused through the control electrode aperture, and subsequently through the first anode aperture. However, when, as described above, the control electrode is removed from the immediate vicinity of the cathode, this focusing action of the modulating electrode is removed from the cathode. The result of this is that the cloud of electrons thermionically emitted from the cathode possesses a random distribution and, under the influence of the accelerating potential of the first anode, only a small proportion of the emitted electrons is directed through the first anode aperture, resulting in low beam current. I have found that this disadvantage may be overcome by adding an additional focusing electrode to the gun in the vicinity of the cathode emissive surface. This additional focusing electrode causes a preliminary focusing of the emitted electrons in the vicinity of the cathode, so that under the influence of the potential of the first anode, a large proportion of emitted electons is focused through the first anode aperture. This produces a high density high perveance beam which may subsequently be modulated by the control electrode located in a field free space between the first and second anodes.

Referring now to Fig. l of the drawing, there is shown a cathode ray tube constructed in accordance with the invention. The cathode ray tube of Fig. 1 comprises an evacuable envelope 1 with an enlarged conical section 2, having a face plate 3 and a luminescent screen 4 at the end thereof, and a narrow neck piece 5 within which electron gun 6 is located. In Fig. 2 the portion of tube 1 enclosing electron gun 6 is shown in vertical crosssection. That portion of electron gun immediately adjacent the cathode is shown in greater detail in Fig. 3. In an illustrative embodiment of the invention, electron gun 6 is shown in Fig. 2 as comprising an indirectly heated cathode cylinder 7, heated by heater wire 8, and having a concave emitting surface 9 which is preferably spherical. Cathode 7 is axially aligned and insulatingly disposed within anode cylinder 11 by insulating disc 10. A hollow cylindrical focusing electrode 12 is axially aligned about the emissive end 9 of cathode 7 and insulatingly disposed within anode cylinder 11 by means of insulating disc 13. Cylindrical anode 11 has a thick apertured end 14 disposed perpendicularly to the axis of cathode 7 and electron gun 6 with a small aperture at the center thereof which may conveniently comprise a cylindrical aperture 17 and a frustro-conical aperture 25 through which are focused the electrons from cathode 7. Aperture 17 is surrounded by an annular, frustro-conical member 14a which projects within the volume defined by focusing electrode 12 in close juxtaposition to cathode ernissive surface 9. A modulating electrode 18 is axially aligned with cathode 7 and anode 11 and perpendicular to the axis thereof and has a small aperture 19 centrally located in axial alignment with the aperture 15 of anode cylinder 11. Modulating electrode 18 is disc-shaped and should have an axial thickness approximately one-half the diameter of aperture 19 located therein. This ratio is important and represents the relationship between control electrode thickness and aperture diameter which yields maximum beam current with the most desirable beam cut-off voltage. If this ratio is made lower than one-half, beam current may be slightly increased; however, beam cut-oif voltage is increased, making proper modulation difiicult. If the ratio is increased above onehalf, beam cut-off voltage decreases, but it becomes necessary to drive the control electrode positive with respect to the cathode in order to maintain high beam current. A positive control electrode, as is Well known, will draw current and is decidedly undesirable.

Immediately following modulating electrode 18 along the axis of electron gun 6, there is located a dished second anode 20 which may be cylindrical in shape with a closed apertured end at its cathode side. This apertured end 21 contains a small aperture 22 which may be approximately /2 the diameter of aperture 19 in modulating electrode 18, and is in axial alignment therewith. The far end of second electrode 20 is open and may be flanged outwardly as shown. Immediately following second electrode 20 there is located, in axial alignment with the other elements of the electron gun, a third anode 23 which is relatively long, cylindrical in shape, and having a first apertured end 24 with an aperture 25 centrally located therein at the cathode end of the cylinder and which may be approximately twice the diameter of aperture 19 in modulating electrode 18, and a second apertured end having an end piece 26 with an aperture 27 therein which may be approximately the same size as aperture 25.

First anode 11, modulating electrode 18, second anode 20, and third anode 23 may be conveniently fastened together, to form a unitary gun structure, by means of a plurality of small rivets 30 spot-welded to the electrodes and embedded within a plurailty of glass or other suitable insulating members 29. The third anode 23 has, at its far end, an annular collar 26 attached thereto to which are connected a plurality of spring fingers 28 which rigidly and centrally locate the electron gun 6 within neck piece 5 of cathode ray tube 1, and additionally connect anode 23 with a conductive coating 30 on the inner surface of the wall 31 of cathode ray tube 1. This conductive coating may conveniently be a colloidal suspension of graphite well known in the art as aquadag, or any similar substance which will form a moderately good conducting film on the interior of the glass envelope. Third anode 23 is maintained at accelerating potential by means of conductive coating 30 and spring fingers 28.

The operation of the illustrative embodiment of the invention as shown in Fig. 2 of the drawing may be explained according to the principles hereinbefore described. Cathode 7 of electron gun 6 is heated to emission temperature by heater wire 8. Electron emissive surface 9 of cathode 7 is coated with an electron-emissive substance,

as for example barium oxide. A cloud of electrons is thermionically emitted from electron emissive surface 9 and fills the region immediately adjacent thereto. Focusing electrode 12 is maintained at a fixed negative potential which may be adjusted between cathode potential and a negative potential of approximately 60 volts. Accelerating anode 11 is maintained at an adjustable positive potential up to a value of approximately 300 volts. The field in the region adjacent emissive surface 9 of cathode 7, under the influence of the potential of face plate 14 of anode 11 and the negative potential of focusing anode 12, attains a concave symmetry and acts as an electron lens to focus the emitted electrons through the small aperture 15 within face plate 14 of anode cylinder 11. A high density, high perveance beam of electrons is accelerated through aperture 15 of face plate 14 and converges to its narrowest point near the center of aperture 19 in modulating electrode 18. The field immediately adjacent cathode emissive surface 9 is shielded from any effects due to modulating electrode 18 by the thick face plate 14 of accelerating anode 11. Thus the maximum density electron beam is focused into the region of the modulating electrode 18. The formed and focused electron beam then passes through aperture 19 in modulating electrode 18. A modulating signal is impressed upon modulating electrode 18 and the field within aperture 19 of electrode 18 operates to modulate the beam current. By making control electrode 13 sufliciently negative with respect to the cathode, the potential at the axis of the beam in the plane of the control electrode may be reduced to zero or slightly below to allow for the thermal velocities of the electrons, in which case all of the beam is reflected back toward the gun. if the potential of the control electrode is now increased slightly, some of the beam current near the axis will be permitted to pass through the control electrode and eventually to reach screen 4 of cathode ray tube 1. As the control electrode is made less negative, the diameter of the area which is above zero potential near the axis of the beam in the plane of the control electrode will increase, allowing a larger fraction of the beam to pass. This modulating is accomplished in a space between equipotential electrodes 14 and 20, which is shielded from external fields and is not affected by the accelerating field in the vicinity of the cathode as is the modulating field in conventional cathode guns.

Once having been modulated at modulator electrode 18, the electron beam passes, under the influence of the strong electric field due to the final accelerating anode 23, through the gap between anodes 20 and 23. This gap constitutes an electron lens which focusses the modulated beam to such a degree that the maximum number of electrons will pass through aperture 27 and reach the screen of the tube. Apertured plate 24 of third anode 23 acts to enhance the focusing action of the electron lens formed by anodes 20 and 23. The diameter of lens aperture 25 should be that diameter which is best suited to give optimum focus to the electron beam at this point, but should yet be large enough to avoid stopping the peripheral electrons in the beam. The effect of this aperture is to shorten the focal length of the electron lens formed between anodes 20 and 23. The electron beam then passes through the far apertured plate 26 of cylindrical accelerating anode 23. Aperture 27 at this end of the anode is approximately equal in size to aperture 25 and acts as a final beam defining aperture which cuts off any stray peripheral electrons from the beam so that the beam will be sharply focused when it impinges upon the face plate 4 of cathode ray tube 1.

In Fig. 4 there is shown a modification of the electron gun 6 in the vicinity of the cathode emissive surface. In this modification, the focusing electrode 12 is in electrical, but not mechanical, contact with cathode 7, and is maintained at all times at cathode potential. It is important that focusing electrode 12 be kept from mechanical contact with cathode 7 to insure thermal insulation.

If focusing electrode 12 is not thermally insulated from cathode 7, it would tend to become slightly emissive also, and the result would be a substantial loss in power and a lowering of the electron gun efficiency. It has been found that with this focusing electrode potential, the optimum field configuration in the vicinity of cathode emissive surface 9 is obtained when annular portion 12a is added to the interior surface of electron focusing electrode 12 such that the aperture between cathode emissive surface 9 and apertured face plate of first anode 11 describes a frustro-conieal shape.

In Fig. 5 of the drawing there is shown in graphical form the improved current characteristics of electron gun 6, when constructed in accordance with the invention. Curve A of Fig. 5, shown as a dotted line, is the current characteristic of a conventional electron gun having approximately the same dimensions as the electron gun constructed according to the invention. As may be seen, the maximum beam current of such a conventional electron gun is approximately only one milliampere for zero control grid bias voltage. More specifically, the conventional gun which gives a beam current characteristic as shown by Curve A has a spacing of 0.005 between cathode and control electrode, a control electrode aperture of 0.035 diameter, a spacing of 0.0l0" between control electrode and first anode, and a first anode aperture diameter of 0.047. The anode is maintained at a potential of 250 v. Curves B, C, D and E of Fig. 5 show the beam current of an improved electron gun constructed according to this invention. These curves were obtained from an electron gun having the following dimensions: cathode diameter, Ms"; first anode aperture, 0.028" diameter; distance between first anode and modulating electrode, 0.030"; modulating electrode thickness, 0,030"; modulating electrode aperture, 0.060" diameter; distance between modulating electrode and second anode, 0.030"; second anode aperture, 0.030" diameter; axial length of second anode, 0.250"; distance between second anode and third anode, 0.07"; third anode apertures, 0.125" diameter; axial length of third anode, 1.125. These values are given only to show the dimensions of the electron gun from which the data for the curves of Fig. 5 were obtained and are not intended to restrict the invention to any particular size electron gun. As may be seen from the curves of Fig. 5, electron beam currents up to 5 milliamperes or five times the maximum beam current for the most nearly comparable conventional electron gun may be achieved depending upon the values assigned to E, the potential of the first and second anodes and E, the potential of the focusing electrode 12 of the improved cathode ray gun.

A further feature of the invention is shown in Fig. 6 of the drawing. In the embodiment of Fig. 6, the hereinbefore described principle of separation of beam forming and modulating functions of the electron gun is utilized to allow a second control electrode to be added to the electron gun to further control the electron beam. In Fig. 6 the cathode 7, focusing electrode 12, first anode 14 and first control electrode 18 are the same as in the electron gun of Fig. 2; however, an intermediate anode 32 and a second control electrode 34 are located between, and in axial alignment Wtih, first control electrode 18 and second anode 20. Intermediate anode 32 comprises a thin disc having a centrally located aperture 33 of approximately the same diameter as aperture 25 in second anode 24. Second control electrode 34 is a disc similar to first control electrode, having an axial aperture 35 therein of the same approximate diameter as first control electrode aperture 19. Intermediate anode 32 is maintained at the same electrical potential as first anode 14 and second anode 20, thus maintaining first control electrode 18 and second control electrode 34 in substantially field free regions so that voltages applied to the control electrodes may operate to modify the electron beam without affecting the electric field at the cathode 9. In the embodiment of Fig. 6, the first control electrode operates to modulate the electron beam as in the embodiment of Fig. 2. Second control electrode 34 may be utilized to impress a second modulating signal to the electron beam independent of the signal applied to first control electrode 18.

One function which may be performed by second control electrode 34 is that of a gating electrode. In this application, a high negative potential is periodically applied to second control electrode 34 so that the electron beam is sharply cut off as the beam is swept by an external sweep system (not shown) from one discrete portion of screen 4 to another discrete portion. Thus a series of sharp images may be produced upon cathodoluminescent screen 4 without exciting intermediate portions of screen 4.

It will be appreciated that, although I have described specific embodiments of the invention, many modifications may be made, and I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

I. An electron gun comprising beam forming means including a cathode with a concave electron emissive surface, a cylindrical beam focusing electrode having a cylindrical inner surface surrounding the cathode emissive surface, said beam focusing electrode being electrically insulated from, and maintained at a fixed potential negative with respect to said cathode, a cylindrical first anode surrounding the cathode and the beam focusing electrode, and having an aperture therein coaxial with the cathode, and an annular protuberance surrounding said aperture and projecting within the volume defined by the focusing electrode and in close proximity to the cathode emissive surface so as to cause the electron field in the vicinity of the cathode emissive surface to be concave away therefrom, said cathode, beam focusing etectrode, and first anode establishing a small diam eter, high density electron beam having high perveance, beam modulating means independent of said beam forming means, comprising a centrally apertured disc beam modulating electrode the thickness of which is approximately one-half the diameter of the aperture therein, aligned with and disposed between the exterior surface of the apertured first anode and a second coaxially aligned anode with an apertured end at the cathode side and an open flanged end away from the cathode, said second anode being electrically connected to the first anode, and means for focusing a modulated beam comprising a third anode with apertured ends coaxially aligned with the cathode and the first and second anodes and forming an electron lens with the flanged end of the second anode, said electron lens further focusing the beam to obtain a high density, high perveance beam.

2. In a cathode ray tube including an evacuable envelope and a cathodoluminescent screen at one end thereof, an electron gun for forming and focusing a beam of electrons upon said cathodoluminescent screen and comprising beam forming means including a cathode with a concave electron emissive surface, a cylindrical beam focusing electrode having a cylindrical inner surface surrounding the cathode emissive surface, said focusing electrode being electrically insulated from, and maintained at a fixed potential negative with respect to said cathode, a cylindrical first anode surrounding the cathode and the beam focusing electrode and having an aperture therein coaxial with the cathode, and an annular protuberance surrounding said aperture and projecting within the volume defined by the focusing electrode and in close proximity to the cathode emissive surface so as to cause the electron field in the vicinity of the cathode emissive surface to be concave away therefrom, said cathode, beam focusing electrode, and first anode establishing a small diameter, high density electron beam having high perveancc, a second coaxially aligned anode with an apertured end at the cathode side, and an open, flanged end away from the cathode, and beam modulating means independent of said beam forming means comprising a plurality of. apertured disc beam modulating electrodes, the thickness of each being approximately /2 the diameter of the aperture therein, aligned with said first and second anodes and separated from one another by a spaced, axially aligned apertured intermediate anode, said first, second and intermediate anode being electrically connected together, and means for focusing the modulated beam comprising a third cylindrical anode with apertured ends coaxially aligned with the cathode and forming an electron lens with the flanged end of the second anode, said electron lens further focusing the beam to obtain a high density, high perveance beam.

3. An electron gun comprising; beam forming means including a cathode having a concave electron emissive surface, a cylindrical beam focusing electrode having a cylindrical inner surface surrounding said cathode emissivc surface and electrically insulated from said cathode, a cylindrical first anode surrounding said cathode and said beam focusing electrode, and having an aperture therein coaxial with said cathode and an annular pro- Luberance surrounding said aperture and protruding within the volume defined by said beam focusing electrode and in close proximity to the cathode emissive surface; means for maintaining said beam focusing electrode at a fixed potential negative with respect to said cathode; means for maintaining said first anode at a positive potential with respect to said cathode, the negative potential of said beam focusing electrode and the postive potential of said first anode cooperating to form an electric field in the vicinity of the cathode emissive surface which is spherically concave away therefrom, said cathode, beam focusing electrode, and first anode establishing a small diameter, high density electron beam having high perveance; beam modulating means independent of said beam forming means comprising a centrally apertured disk beam modulating electrode, the thickness of which is approximately one-half the diameter of the aperture therein, aligned with and disposed between the exterior surface of said apertured first anode and a second coaxially aligned anode with an apertured end at the cathode side and an open flange away from the cathode; said second anode being electrically connected to said first anode; and means for focusing a modulated beam comprising a third anode With apertured end coaxially aligned with said cathode and said first and second anodes and forming an electron lens with the flanged end of said second anode, said electron lens further focusing the beam to obtain a high density, high perveance beam.

References Cited in the file of this patent UNITED STATES PATENTS 2,153,269 Nicoll Apr. 4, 1939 2,190,515 Hahn Feb. 13, 1940 2,194,380 Broadway et al Mar. 19, 1940 2,199,540 Diels May 7, 1940 2,301,490 Winans Nov. 10, 1942 2,355,795 Glass Aug. 15, 1944 2,389,903 Hahn Nov. 27, 1945 2,591,689 Field Apr. 8, 1952 2,672,568 Cole et al. Mar. 16, I954 

